Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as comprising two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pulse generator to the heart. A lead may provide both unipolar and bipolar pacing polarity electrode configurations. In unipolar pacing, the pacing stimulation pulses are applied between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In bipolar pacing, the pacing stimulation pulses are applied between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Traditionally, therapy delivery had been limited to the venous, or right side of the heart. The reason for this is that implanted electrodes can cause blood clot formation in some patients. If a blood clot were released arterially from the left heart, as for example the left ventricle, it could pass directly to the brain potentially resulting in a paralyzing or fatal stroke. However, a blood clot released from the right heart, as from the right ventricle, would pass into the lungs where the filtering action of the lungs would prevent a fatal or debilitating embolism in the brain.
Recently, new lead structures and methods have been proposed and even practiced for delivering cardiac rhythm management therapy to the left heart. These lead structures and methods avoid direct electrode placement within the left atrium and left ventricle of the heart by lead implantation within the coronary sinus region of the heart. As used herein, the phrase “coronary sinus region” refers to the vasculature of the left ventricle, including any portions of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.
It has been demonstrated that electrodes placed in the coronary sinus region of the heart may be used for left atrial pacing, left ventricular pacing, or cardioversion and defibrillation. These advancements enable implantable cardiac stimulation devices to address the needs of a patient population with left ventricular dysfunction and/or congestive heart failure which would benefit from left heart side pacing, either alone or in conjunction with right heart side pacing (bi-chamber pacing), and/or defibrillation.
While left heart side pacing represents a significant advancement for those patients which require such therapy, it itself is not without its own challenges. One such challenge relates to capture threshold. More specifically, it has been found that the capture thresholds, which must be exceeded to effectively pace the heart, are generally higher for left heart side pacing then for right heart side pacing. Interestingly, the potential complication is not in the ability of a device to have sufficient output to capture the particular heart chamber, but the possibility of stimulating muscle tissue other than the desired heart muscle in the process.
The other muscle tissue mentioned above will be referred to herein as parasitic muscle tissue which includes any muscle tissue other than the muscle tissue of the particular heart chamber to be captured by the pacing pulses. Typically the parasitic muscle tissue stimulation would be chest muscle, but other non-cardiac muscle tissue may also be involved.
It is now common for pacemakers to have automatic capture determination functionality for automatically setting the pacing pulse output amplitude (current or voltage) to a level required to exceed a heart chamber pacing threshold to capture and thus effectively pace the heart chamber. Typically, the determination is made over a range of output levels at a constant pulse duration until the capture threshold is found. The device output is then set to the threshold plus an added safety margin. A constant pulse duration is used because the capture threshold voltage or current varies with pulse duration.
The stimulation threshold of parasitic muscle tissue generally does not vary with pulse duration and is substantially constant. For capture thresholds in a normal range, the output amplitude to which the device is automatically set essentially never exceeds the stimulation threshold of parasitic muscle tissue. However, for higher capture thresholds, and especially those of left heart side pacing, the automatically set device output can not only exceed the pacing capture threshold, but the parasitic muscle tissue stimulation threshold as well. As a result, the present invention addresses this issue.